Expansion and degassing device for connecting to a circut system, in particular the circuit system of a building heating installation

ABSTRACT

An expansion and degassing container is in an expansion and degassing device for connecting to a circuit system for a circulating liquid. Because the amount of circulating liquid in the circuit system constantly changes during operation, liquid is withdrawn from the expansion and degassing container by way of a pressure holding pump, or supplied via a supply line, if needed. The supply line for supplying the refilling liquid is provided as a closed line, a system separator being arranged at the beginning region of the line and the end region thereof penetrating the container cover of the expansion and degassing container in a pressure-tight manner. The valve of the supply line is controlled according to the liquid level in the container. A pressure load cell, which additionally controls the valve according to the pressure, is present as a unit for decreasing an undesirable level of negative pressure.

BACKGROUND OF THE INVENTION

The invention relates to an expansion and degassing device forconnecting to a circuit system, in which a circulating liquid is pumpedin a loop and undergoes pressure and volume changes. Such a device isknown from the Swiss patent specification CH 694 895 A5.

The circuit systems that may be used for expansion and degassing devicesof the type referred to herein are described as a cooling circuit systemand a heating circuit in CH 694 895 A5. In these cases, the circulatingliquid also undergoes temperature changes. As a result of the heatingand cooling of the circulating liquid during operation, or optionally bythe activation and deactivation of individual loads that are chargedwith the circulating liquid, the pressure and volume of the circulatingliquid change constantly during operation. Consequently, a volumetriccontent of the circulating liquid must be constantly withdrawn orreplenished, and the required optimal operating pressure must bemaintained. For this purpose, the known expansion and degassing devicehas a basically closed and rigid expansion and degassing container,which serves for intermediate storage and receives or releasesfluctuating amounts of the circulating liquid. The expansion anddegassing container is additionally used for functions such asmaintaining the pressure in the system, degassing the circulatingliquid, and feeding refilling liquid, as well as a settling tank forsalts and minerals. To the extent possible, contact with the atmosphericoxygen is to be avoided during all these functions, because this causesthe entire installation to corrode.

The connection between the circuit system and the expansion anddegassing device is established by a bypass or parallel line in apartial flow, which in CH 694 895 A5 is referred to as a “circulationline.” This line connects the flow line of the circuit system to thereturn line thereof, so that the circulating liquid likewise constantlyflows in a loop in the bypass or circulation line.

A container supply line, which leads into the interior of the expansionand degassing container, is connected to a first connecting point of thebypass or circulation line via an overflow valve. Downstream of thisfirst connecting point, the bypass or circulation line has a secondconnecting point into which the discharge port of a pressure holdingpump leads. The intake line of the pressure holding pump likewiseextends into the interior of the expansion and degassing container. Theinterior of the expansion and degassing container is thus connected byway of the pressure holding pump to a first point, and by way of theoverflow valve to a second point, of the circulating liquid of thecircuit system that is pumped in a loop. Excess amounts of thecirculating liquid flow into the expansion and degassing container viathe overflow valve and via the container supply line; conversely, ifadditional amounts of circulating liquid are required in the circuitsystem so as to maintain the pressure, these amounts are withdrawn fromthe expansion and degassing container by the pressure holding pump.

This constant change results in pressure fluctuations in thesubstantially closed expansion and degassing container, whereinpredetermined positive and negative pressures are to be reached. Thepressure in the interior of the expansion and degassing container istypically slightly reduced as compared to the outside atmosphericpressure. The container interior can only be connected to the outsideatmosphere by way of pressure control valves. For this purpose, an airbleed and safety valve (gas drain valve) is provided in CH 694 895 A5.When gases have collected in the interior of the expansion and degassingcontainer and apply a particular pressure, the air bleed and safetyvalve opens and allows these gases to escape through a water seal intothe atmosphere. As a result, neither the circulating liquid of thecircuit system nor the fresh refilling liquid comes in undesirablecontact with the gases to be discharged. The outflow of the gases alsocauses a pressure equalization and results in the closing of the airbleed and safety valve. At this point, no outside air can thus flow intothe expansion and degassing container. The air bleed and safety valvecan have a simple mechanical construction and control; optionally,liquid components may also leave the container interior via this valve.The expansion and degassing container can be filled with liquid to thelevel of the air bleed and safety valve.

If the level of liquid in the interior of the expansion and degassingcontainer has dropped to a particular lower level, fresh refillingliquid must be replenished. According to CH 694 895 A5, the solenoidvalve of a supply line is opened for this purpose by the controlelectronics when a lower level switch is activated. This line ends withfree outflow via a float chamber, which is arranged on the containercover of the expansion and degassing container. The refilling liquidthus flows as a free stream into the float chamber, can degas into theambient air, and collects in the float chamber to a particular height,at which the float control is activated and allows the refilling liquidto flow downward into the expansion and degassing container. Theinterruption of the supply line by a section with a free stream alsoserves the purpose of preventing container liquid from flowing back intothe supply line of the refilling liquid. However, the outflow of therefilling liquid in a free stream has the disadvantage that oxygen fromthe atmosphere can be taken up. Another disadvantage of the known designrelates to the float control, because air can likewise enter therefilling liquid via the valve that is controlled by the float. This isin particular the case when the movable valve body, which generally is avalve disk, lifts off or is lifted off the seat thereof.

According to CH 694 895 A5, provisions have already been made to theeffect that impermissibly high negative pressure occurs in the expansionand degassing container. In extreme cases, the container could thenimplode. According to CH 694 895 A5, a so-called vacuum breaker is thusarranged on the container cover of the expansion and degassingcontainer, which is to say a valve that establishes a connection to theoutside atmosphere when a limit value for an impermissibly high negativepressure is reached. While the inflow opening of the vacuum breaker isagain located inside a plumbing unit, this does not change the fact thatair can find its way into the interior of the expansion and degassingcontainer when the vacuum breaker is activated. According to CH 694 895A5, this must be tolerated because the vacuum breaker is only providedfor disturbances that occur as exceptions.

In addition, expansion and degassing devices are known, which operatewith pressure holding pumps and in which the closed expansion anddegassing container is provided with a diaphragm. The diaphragm dividesthe interior of the container into a gas compartment and a liquidcompartment. For example, the diaphragm can be designed in the manner ofa sack, which contains the liquid and is located inside the expansionand degassing container. An intermediate space, which receives a gas,this being air for example, is formed between the outer side of the sackand the inside wall of the expansion and degassing container. If thecirculating liquid expands when the temperature increases, the sackforming the diaphragm absorbs the change in volume with the air. Thedrawbacks that arise due to the pressure control valves in the devicecomprising the rigid expansion and degassing container according to CH694 895 A5 are thus avoided. However, the material of which thediaphragms are made likewise causes problems. The diaphragm material canbe rubber or butyl rubber, for example. If these materials are inconstant contact with certain circulating liquids, such as heating waterfor example, and subject to constant strong deformation, materialfatigue may result. Gases, including oxygen, can diffuse through thediaphragm into the circulating medium of the pipe network, and thusthere is a further risk of corrosion and silting. The pipe networks mustbe bled more frequently as a preventive measure, which incurs additionalcosts. Because the deformability of the diaphragms is limited, thepressure range between the positive pressure and negative pressure thatan operating program can cover is also limited. Finally, the diaphragmsmust also be replaced more frequently, in accordance with pressurecontainer regulations, so as to assure flawless functioning. This isbecause the temperatures cause the plasticizer to evaporate, and therubber material becomes porous and decomposes.

SUMMARY OF THE INVENTION

Thus, it is the object of the invention to improve the expansion anddegassing device of the type mentioned above, with the aim that the samecan be operated over a relatively large pressure range between positivepressure and negative pressure in the expansion and degassing container,yet has a simple and reliably functioning construction, that theabsorption of oxygen from the atmosphere into the container iscompletely excluded, and that the backflow of container liquid into therefilling liquid is also safely prevented (system separation).

The overflow valve is designed so that the medium to be degassed flowsfrom the heating circuit into the degassing container, and moreparticularly in accordance with the pressure in the container that isdesired during normal operation and can be set at the overflow valve.This desired pressure (by way of example) is 0.8 to 0.9 bar and is thusslightly below the outside atmospheric pressure of 1 bar, the pressuredifferential at which the overflow valve opens being set in keeping withthe static positive pressure of 2 bar (by way of example) that ispresent at this point of the heating system. This proven characteristichas been derived from the generic prior art according to CH 694 895 A5.

So as to completely exclude the absorption of oxygen, two modificationsare made to the container (characterizing features a and b) over thedisclosure of CH 694 895 A5. First, the vacuum breaker, the purpose ofwhich was to prevent excessive negative pressure in the degassingcontainer by the supply of air (or supply of gas in general), iseliminated; the function thereof is performed in another manner, andmore particularly by the supply of fresh water (feature b).

Previously, according to the disclosure of CH 694 895 A5, the supply offresh water was carried out so that the fresh water valve, which islocated in the “open” line of the fresh water replenishment system, wasonly controlled according to the level. The novel supply of fresh wateris now carried out so that the solenoid valve is located in a “closed”supply line (feature a) and opens not only according to the level, butalso according to the pressure (feature b). As a result, a vacuumbreaker, which allows either atmospheric air or inert gas (in any case agaseous medium) to flow in, can be dispensed with. An essential part ofthe invention is thus that, instead of a gas, a fluid or liquid mediumis used with the fresh water supply so as to limit the negativepressure. Because the vacuum breaker is eliminated, and also because thefresh water supply line is no longer connected to the atmosphere, thedegassing container is completely protected from all oxygen.

As a result, the negative pressure in the expansion and degassingcontainer is limited (i) by opening the overflow valve at a relativelylow negative pressure relative to the atmosphere and (ii) by way of anovel negative pressure load cell, which can open the fresh watersolenoid valve that is already present elsewhere. In an olderapplication according to DE 10 2010 047 514, the fresh water wasreplenished by way of the solenoid valve in a closed supply line,wherein the solenoid valve was only controlled according to the level.Instead of the “open” vacuum breaker according to CH 694 895 A5, asafety valve comprising a connected inert gas container was used in DE10 2010 047 514. This also represented an already successful solution,which in the particular operating states of replenishment and negativepressure limitation replaced the connections that were previously opento the atmosphere with the respective closed connections. However, this“closed” solution was more complex than the “open” solution in thepresent example. It was found in the invention that a supply of nitrogencan also be dispensed with.

Due to the design of the expansion and degassing container, which is tosay, with such oxygen degassing, an oxygen content of less than 0.1mg/l, typically 0.05 mg/l, is achieved in the circulating medium of thesystem. The introduction of oxygen due to practically inevitable leaksin the heating circuit and the oxygen that is contained in the likewiseinevitable supply of fresh water are reliably removed from the system bythe degassing in the container before these can contribute to corrosion.

Because the refilling liquid in the now closed supply line flows throughthe container cover into the interior of the closed and pressure-tightexpansion and degassing container, any contact between the circulatingliquid and the free atmosphere at this point is excluded, and thedegassing and deliming take place only within the container. Thus,undesirable corrosion by atmospheric oxygen is limited at this point.

The system separator at the inlet of the supply line is constructed inaccordance with DVGW (German Technical and Scientific Association forGas and Water) guidelines and thus is a reliable component. Thiscomponent is certain to prevent backflowing container liquid from mixingwith fresh refilling liquid. This is because, when the pressure in thesupply line downstream of the system separator rises in comparison withthe pressure at the inlet port, and exceeds a particular limit value,the inlet-side backflow preventer of the system separator closes andconducts container liquid that has penetrated into the supply line, forexample, through a secondary line to a collection site, for example aspillover siphon.

However, the entire advantageous effect of the expansion and degassingdevice according to the invention is also achieved by precluding anypenetration of atmospheric air into the interior of the expansion anddegassing container during the necessary function of vacuum breaking(this being the function of limiting the negative pressure in thecontainer). The negative pressure must be limited so as to prevent thecontainer liquid from boiling and evaporating and so that a massivedesign is not required for the container. If the negative pressure inthe expansion and degassing container becomes impermissibly high, aliquid medium flows into the upper region of the container.

Because a negative pressure exists, generally only a very small amountof gas is taken up by the container liquid. In addition, pressureequalization takes place very rapidly. Any excess gas leaves theexpansion and degassing container via the air bleed and safety valve(gas drain valve) already mentioned in connection with the prior art, assoon as the pressure is once again high enough in the expansion anddegassing container.

A major advantage of the expansion and degassing device according to theinvention is that it operates without diaphragms. This prevents thedifficulties that result from the materials of the membranes being proneto wear. The expansion and degassing device according to the inventionpreserves the proven pressure-tight and rigid expansion and degassingcontainer and operates with proven valves that generally have a simplemechanical function. In addition, this allows the device according tothe invention to be operated over a wide pressure range between positivepressure and negative pressure in the expansion and degassing container.The modular design of the device, which is proposed in CH 694 895 A5,can be preserved.

It is possible with the expansion and degassing device according to theinvention to expel all gases, such as hydrogen, nitrogen, oxygen, sulfurhydrocarbons and carbon dioxide, from the circulating liquid. An inertfluid develops in the heating circuit, wherein the pH value risescompared to the replenishment liquid and the electric conductance valuedecreases. The pH value should be between 8.2 and 10.0 and theelectrical conductivity should be less than 100 microsiemens per cm fora low-salt medium. As a result, corrosion by oxygen, which is at aconcentration of less than 0.1 mg/l, is negligible.

Maintaining the pressure in the heating circuit likewise means that thecirculating liquid must be constantly undersaturated, so as to allow forthe uptake of substances and so that the pipes and walls of theinstallation are cleaned of substances.

Degassing also provides improvement in the heat transfer in the circuitsystem, so that primary energy is saved, and thus less carbon dioxide isreleased.

Moreover, the substantial degassing of the water or medium has theadvantage that the expansion of the water under the influence of thetemperature is lower (comparable to mercury), whereby the pressurefluctuation that must be compensated for is also lower.

Advantageous refinements are described in the dependent claims, and thecharacteristics claimed therein are described in more detail in theexemplary embodiment.

According to claim 7, the invention also relates to the circuit systemof a building heating installation, in which circulating water is pumpedin a loop and undergoes pressure and volume changes, wherein theinvention in this field of application is that of providing the buildingheating installation with an expansion and degassing device according toat least one of claims 1 to 6 so as to treat the circulating water.Claim 8 contains the corresponding design for a cooling circuit system.

In the case of the building heating installation, the refilling liquidis fresh water, and the circulating water is treated in a particularmanner for heating purposes. The device according to the invention isparticularly well suited for the purposes of building heatinginstallations in the low temperature range, and also in the range of thecooling systems, because these operate in temperature ranges in whichthe density of the water increases and the absorbing power rises.However, a number of other fields of application, such as refrigerationcircuits, are also possible.

The invention will be described in more detail hereafter based on oneexemplary embodiment in three figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a schematic illustration ofthe expansion and degassing device according to the invention;

FIG. 2 shows an enlarged detail of FIG. 1; and

FIG. 3 shows a diagram similar to that of FIG. 1, comprising anassociated electronic control region.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, reference numeral 1 denotes the collectivity of an expansionand degassing container. The container 1 comprises the container wall 2,which forms the cylindrical lateral wall and can be made of stainlesssteel. The container cover 3 and the container bottom 4 are likewisemade of stainless steel and are detachably fastened to the containerwall 2. The container wall 2 can additionally be surrounded by a casing5 made of a reflective and insulating material. Details of this casing 5have already been described in the aforementioned CH 694 895 A5 or in DE103 29 740 A1. In addition, a drain valve 6 is provided in the containerbottom 4. So as to monitor the level of the liquid in the expansion anddegassing container 1, a level indicator is present thereon, which canbe a glass gauge 7. A floating body can trigger particular switch andcontrol functions by reaching an upper level switch 8 or a lower levelswitch 9. Moreover, a closable inspection opening 10 is present, whichis used for cleaning and repair work.

An intake line 11 penetrates the container cover 3. The line ends in acheck valve 12 in the region of the bottom third of the expansion anddegassing container 1. The check valve 12 ensures that, duringoperation, only liquid from the lower region is taken in; liquid should,however, be prevented from flowing back downward into the container 1.The intake line 11 leads to a pressure holding pump 13. The outlet ofthe pressure holding pump 13 on the pressure side leads into acirculation line (16, 17), which is connected to the circuit system ofthe entire installation as a bypass or parallel line; for details, againrefer to CH 694 895 A5 (refer to FIG. 4 in particular). A supply line 16of the expansion and degassing device is connected to the pressure lineof the circuit system, while a return line 17 of the expansion anddegassing device is connected to the return line of the circuit system.The supply line 16 and the return line 17 thus together form thecirculation line. The flow directions in the lines 16 and 17 areindicated by the directional arrows 16 a and 17 a. A connecting line 18leads from the supply line 16 in a first branch 18 a to an overflowvalve 15, which is provided at the upper end of a container supply line14. A connection for a pressure gage 19 and/or a pressure control device19 or the like is also provided in the first branch 18 a of theconnecting line 18. A second branch 18 b conducts the pressure and issecured by a tamper-proof valve. This valve is used to allow thepressure and function of the container 1 to be set, without impactingthe circulation line. The second branch 18 b of the connecting line 18forms the classic expansion line, and is integrated in the dynamic zeropoint of the system. The static pressure of the system is made availableby the second branch 18 b, via which the excess amounts of circulatingliquid are conducted into the expansion and degassing container 1 usingthe pressure-controlled, adjustable overflow valve 15. Static anddynamic pressures are present in the lines 16 and 17. The pressures inthe lines 16, 17 and 18 are equalized by way of the connecting line 18.

The line section comprising the connecting line 18 and the first branch18 a is also used for pressure equalization and is therefore alsoreferred to as the expansion line. If, during operation of the circuitsystem, the circulating liquid circulating therein expands, theresulting pressure increase in the connecting or expansion line 18causes the overflow valve 15 to open and the additional volume to bedischarged via the container supply line 14 to the expansion anddegassing container 1. The overflow valve 15 is designed as adifferential pressure valve and is protected from pollution by a screen.The screen retains coarse particles, which can be washed up from theliquid circuit, while small particles are flushed through withoutdifficulty because of the pressure gradient between the operatingpressure in the return line 17 and the significantly lower internalpressure in the expansion and degassing container 1. The containersupply line 14 having the air outlet 14 a already allows gases to bereleased, wherein the air outlet 14 a is located in the upper region ofthe container supply line 14 in a regionally widened area (partialhousing).

Advantageously, a safety valve, with or without a nitrogen bottle, is nolonger required at the container cover 3.

Another assembly relates to the supply and removal of supplemental orrefilling liquid into and from the expansion and degassing container 1.For improved clarity, the enlarged illustration according to FIG. 2 wasselected for this purpose. This shows the case where the circuit systemis a heating system, in which the circulating liquid is speciallytreated water. The refilling liquid is fresh water in this case. Thesupply takes place via a closed fresh water supply line 23, which iscollectively denoted by 23 and the end region 23 a of which is a rigidpipeline that penetrates the container cover 3 in a pressure-tightmanner. This fresh water supply line 23 is connected to a systemseparator (mechanical disconnector) 24 by way of a solenoid valve 27.The connecting line between the system separator 24 and the solenoidvalve 27 is composed of a flexible line section 25 and an intermediatepiece 26. The fresh water flows into the system separator 24 via theinlet port 24 a thereof. Such system separators are known in terms ofthe mode of operation thereof and are available on the market asready-to-install units for additional plant engineering components. Suchsystem separators are composed of two backflow preventers that areconnected in series and equipped with a ventable center zone. Thefunction thereof in the expansion and degassing device according to theinvention will be described hereafter.

The latter assembly also includes an air bleed valve 30, which isprovided on the container cover 3 and can establish a connection betweenthe container interior and the outside atmosphere. The air bleed valveis based on the function of a flap plate 31, which lifts under theaction of slight positive pressure in the interior of the expansion anddegassing container 1 and thus allows the positive pressure to bereleased. Excess air can thus escape from the container interior to theoutside, as can excess amounts of liquid, if the liquid inadvertentlyreaches the inside of the container cover 3. The guided flap 31 is thenlikewise lifted and allows this excess liquid to flow into anintermediate chamber 32.

A first spillover line 28 leads from the system separator 24 to aspillover siphon 29. Moreover, a second spillover line 33 leads from theintermediate chamber 32 likewise to the spillover siphon 29.

The described expansion and degassing device operates as follows:

It shall be assumed again that the circuit system is a building heatinginstallation and that the water in the expansion and degassing container1 is approximately at the level of the region limit 34. A settling zoneA is formed there, in which salts and suspended oxide particles orsolids, notably lime, are deposited. Here, the intent is to supplyadditional fresh water to the expansion and degassing container 1. Thiswill automatically take place no later than when the lower level switch9 is activated. The solenoid valve 27 will then open.

The fresh water flows via the system separator 24, the flexible linesection 25 and the intermediate piece 26 through the solenoid valve 27,and finally via the end region 23 a, directly into the expansion anddegassing container 1, without coming in contact with atmosphericoxygen. During the inflow, an expansion region E, and beneath the same asmoothing region B, form in the expansion and degassing container 1, seealso the region limit 35, as is described in detail in CH 694 895 A5.The inflowing fresh water, as well as the water that is already presentin the container, degas in the vacuum or in the negative pressure of theexpansion region E, so that substantially all gases escape.

Meanwhile the smoothing region B exerts a piston action, as a result ofwhich the pressure in the expansion region E rises when the level ofliquid in the expansion and degassing container 1 increases, whereby theslight negative pressure initially present in the expansion anddegassing container is ultimately eliminated. A slight positive pressurelifts the guided flap 31 of the air bleed valve 30, so that the air andgas components present in the container 1 escape into the intermediatechamber 32 and thus transition into the outside atmosphere, withoutcoming in contact again with the fresh water or the circulating water ofthe circuit.

At the same time, the escaping of air and gas components from theexpansion and degassing container 1 results in pressure equalization, sothat the flap 31 closes tightly again under the influence of gravity.Thus, no oxygen-containing outside air reaches the interior of theexpansion and degassing container 1.

During the operation of the circuit system, the circulating water willgradually warm and consequently expand. This leads to a pressureincrease in the secondary circuit connected in parallel comprising thesupply line 16 and the return line 17, with this increase acting on theoverflow valve 15 via the connecting or expansion line 18 and the firstbranch 18 a. This overflow valve opens at a defined limit value andallows the excess circulating water to flow through the container supplyline 14 downward into the expansion and degassing container 1. There,the gases contained in the fluid escape, as was already described forthe fresh water. The level of liquid in the expansion and degassingcontainer 1 thus continues to rise.

It is possible for the amount of water that is present in the expansionand degassing container 1 to rise unexpectedly from the level of thelower level switch 9 to the container cover 3. In the fixed fresh watersupply line 23, this may lead to a gradual back pressure, which acts onthe system separator 24 and causes the same to be activated. In thiscase, the system separator 24 will block the supply of fresh water inkeeping with the function thereof and open to the first spillover line28. The water coming from the expansion and degassing container 1 thusdrains via the first spillover line 28 into the spillover siphon 29. Thesystem separator 24 thus protects the fresh water present at the inletport thereof from the liquid coming from the expansion and degassingcontainer 1, and this liquid can no longer flow into the fresh waternetwork.

In contrast, if the circulating water present in the circuit systemcools, this takes up less volume, so that the pressure control device 19is activated and the electronic system switches on the pressure holdingpump 13. The amount lacking in the system is thus replenished and theoperating pressure is maintained in the circuit system. The check valve12 consequently opens, and the water present in the expansion anddegassing container 1 is supplied via the intake line 11 to the returnline 17. This consumption can be very high, so that the water level inthe expansion and degassing container 1 drops very quickly to the lowerlevel switch 9. This creates a significant negative pressure in theexpansion and degassing region E. There is the risk that the containermay implode. In addition, the fluid in the settling region A willpartially evaporate under highly negative pressure.

So as to prevent the risk of implosion and boiling, the solenoid valve27 in the fresh water connection opens when the negative pressure is toohigh. As a result, refilling liquid flows into the expansion anddegassing container 1. This also prevents the interior of the expansionand degassing container 1 from coming in contact with the atmosphericoxygen. The inflow of liquid into the expansion region E causes pressureequalization and thus limitation of excessively high negative pressure.This prevents the possible implosion of the container 1, without thewater present in the container 1 making contact with the oxygen. Incontrast to the widely common prior art, a diaphragm is not required.Under negative pressure, the gases escape even more readily from theheating water in the settling region A and the smoothing region B.Swirling of the water results in even better degassing.

In the exemplary embodiment according to FIGS. 1 and 2, the pressuremeasurement signal of the negative pressure load cell 36 acts directlyon the solenoid valve 27, so as to open the same if necessary. Thisnegative pressure function takes priority over the conventional levelfunction that is initiated by the lower level switch 9, which is to saywhenever the negative pressure signal (of 36) requires opening and, atthe same time, the level signal (of 9) prefers closing, the solenoidvalve 27 is opened. Direct actuation can take place by way of a 220 voltsignal, to which the switch elements are adjusted. If too much mediumenters the entire system during a priority process, an excess amountflows off via the siphon 29.

The novel negative pressure function also takes priority overmaintaining the pressure in the system and optionally causes thepressure holding pump 13 to stop.

As an alternative, FIG. 3 shows an electronic control unit 37, which islocated in an electronic control region A2, including the network cableconnection 38. The alternative function of the same is to convert themeasurement signal of the negative pressure load cell 36 into an openingsignal for the solenoid valve 27 in the fresh water replenishment (23 to27), again with priority over the level measurement signal. In thisalternative it is also possible to use 220 volt signals, either entirelyor in part, as control signals. The priority switching in thisalternative can take place inside the control unit 37, wherein the twocontrol signals advantageously can also be weighted against each other.

1. An expansion and degassing device for connecting to a circuit system,the circuit system comprising a loop, including a return line, in whicha circulating liquid is pumped and undergoes pressure and volumechanges, the expansion and degassing device comprising a closedexpansion and degassing container having a container cover, a pressureholding pump via which the container is connected to a first point inthe loop, an overflow valve via which the container is connected to asecond point of the loop, an air bleed and safety valve, a unit forsupplying refilling liquid as needed to maintain liquid in the containerat predetermined levels, a unit for decreasing negative pressure in theexpansion and degassing container, a closed supply line of the unit forsupplying the refilling liquid, and a system separator of the unitprovided in a beginning region of the closed supply line, wherein theclosed supply line in an end region thereof penetrates the containercover of the expansion and degassing container in a pressure-tightmanner, wherein the system separator opens a delivery path when acritical pressure increase occurs in the closed supply line, with apressure increase starting from the end region, wherein the deliverypath leads past the expansion and degassing device, and wherein the unitfurther comprises a pressure load cell which actuates the unit so as tosupply refilling liquid for the purpose of reducing the negativepressure to below a limit value.
 2. The expansion and degassing deviceaccording to claim 1, further comprising a circulation line to which thepressure holding pump and the overflow valve are connected, thecirculation line comprising a circulation line supply line and acirculation line return line, and wherein the supply line is connectedto the first point and the return line is connected to the second pointof the loop.
 3. The expansion and degassing device according to claim 1,further comprising a circulation line to which the pressure holding pumpand the overflow valve are connected, the circulation line comprising acirculation line supply line and a circulation line return line, and thecirculation line supply line and the circulation line return line beingconnected at a distance from each other to two points of the return lineof the circuit system.
 4. An expansion and degassing device according toclaim 1, wherein the system separator comprises a three-chamber systemhaving controllable inlet pressure, middle pressure and back pressurezones, so that liquid flowing back into the closed supply line from theexpansion and degassing container is completely discharged to acollection point.
 5. The expansion and degassing device according toclaim 4, wherein the collection point is a spillover siphon.
 6. Anexpansion and degassing device according to claim 1, wherein liquid andgaseous layers thereabove in the expansion and degassing containerimmediately adjoin each other without separation.
 7. An expansion anddegassing device according to claim 1, wherein the unit for decreasingnegative pressure includes a solenoid valve through which the refillingliquid is supplied, a lower level switch is situated in the container,which switch is actuated for emission of a signal upon liquid level inthe container falling below a predetermined level, and a pressuremeasurement signal from the pressure load cell as a control signal ofthe solenoid valve takes priority over a level measurement signal of thelower level switch as a control signal of the solenoid valve.
 8. Theexpansion and degassing device according to claim 7, further comprisingelectronic controls via which a signal of the pressure load cell acts onthe solenoid valve.
 9. The expansion and degassing device according toclaim 7, wherein a signal of the pressure load cell acts directly on thesolenoid valve.
 10. The expansion and degassing device according toclaim 7, wherein the signal of the pressure load cell is a 220 voltswitch signal which matches a 220 volt signal of the solenoid valve. 11.A circuit system of a building heating installation, in whichcirculating water is pumped in a loop and undergoes pressure and volumechanges, combination with an expansion and degassing device according toclaim 1 which expansion and degassing device is connected to the circuitsystem of the building heating installation.
 12. A cooling circuitsystem, in which cooling liquid is pumped in a loop and undergoespressure and volume changes, in combination with an expansion anddegassing device according to claim 1 which expansion and degassingdevice is connected to the cooling circuit system.
 13. The expansion anddegassing device according to claim 8, wherein the signal of thepressure load cell is a 220 volt switch signal which matches a 220 voltsignal of the solenoid valve.